Computerized Tomography (CT)

Computed tomography (CT) uses sophisticated x-ray technology to help detect a variety of diseases and conditions. CT scanning is fast, painless, noninvasive and accurate. In emergency cases, it can reveal internal injuries and bleeding quickly enough to help save lives.

Tell your doctor if there's a possibility you are pregnant and discuss any recent illnesses, medical conditions, medications you're taking, and allergies. You will be instructed not to eat or drink anything for a few hours before your test.

If you have a known allergy to contrast material, your doctor may prescribe medications to reduce the risk of an allergic reaction. Leave jewelry at home and wear loose, comfortable clothing. You may be asked to wear a gown.

WHAT IS A CT SCAN?

Computed tomography, more commonly known as a CT or CAT scan, is a diagnostic medical imaging test and it produces pictures of the inside of the body.

The cross-sectional images generated during a CT scan can be reformatted in multiple planes and even generate three- dimensional images.

CT images of internal organs, bones, soft tissue and blood vessels provide greater detail than traditional x-rays.

Using specialized equipment radiologists can more easily diagnose problems such as cancer, cardiovascular disease, infectious disease, trauma and musculoskeletal disorders.

WHAT ARE SOME COMMON USES OF THE PROCEDURE?

CT imaging is:

• one of the fastest and most accurate tools for examining the head, chest, abdomen and pelvis because it provides detailed, cross-sectional views of all types of tissue
• used to examine patients with injuries from trauma such as a motor vehicle accident
• performed on patients with acute symptoms
• often the best method for detecting cancers. It's considered the best method since the image allows a physician to confirm the presence of a tumor, measure its size, identify its precise location and determine the extent of its involvement with other nearby tissue
• an examination that plays a significant role in the detection, diagnosis and treatment of vascular diseases that can lead to stroke, kidney failure or even death

Radiologists and radiation oncologists often use the CT examination to:

• Quickly identify injuries to the body in cases of trauma
• Guide biopsies and other procedures such as abscess drainages and minimally invasive tumor treatments
• Plan for and assess the results of surgery
• Stage, plan and properly administer radiation treatments for tumors as well as monitor response to chemotherapy
• Measure bone mineral density for the detection of osteoporosis

HOW SHOULD I PREPARE?

You should wear comfortable, loose-fitting clothing to your exam. You may need to wear a gown during the procedure.

Metal objects, including jewelry, eyeglasses, dentures and hairpins, may affect the CT images. Leave them at home or remove them prior to your exam. You may also be asked to remove hearing aids and removable dental work. Women will be asked to remove bras containing metal underwire. You may be asked to remove any piercings, if possible.

You will be asked not to eat or drink anything for a few hours beforehand, if contrast material will be used in your exam. You should inform your physician of all medications you are taking and if you have any allergies. If you have a known allergy to contrast material, your doctor may prescribe medications (usually a steroid) to reduce the risk of an allergic reaction. To avoid unnecessary delays, contact your doctor before the exact time of your exam.

Also inform your doctor of any recent illnesses or other medical conditions and whether you have a history of heart disease, asthma, diabetes, kidney disease or thyroid problems. Any of these conditions may increase the risk of an adverse effect.

Women should always inform their physician and the CT technologist if there is any possibility that they may be pregnant.

WHAT DOES THE CT EQUIPMENT LOOK LIKE?

The CT scanner is typically a large, donut-shaped machine with a short tunnel in the center. You will lie on a narrow examination table that slides in and out of this short tunnel. Rotating around you, the x-ray tube and electronic x-ray detectors are located opposite each other in a ring, called a gantry. The computer workstation that processes the imaging information is located in a separate control room. This is where the technologist operates the scanner and monitors your exam through direct visual contact. The technologist will be able to hear and talk to you using a speaker and microphone.

HOW DOES THE PROCEDURE WORK?

In many ways, a CT scan works like other x-ray exams. Different body parts absorb x-rays in different amounts. This difference allows the doctor to distinguish body parts from one another on an x-ray or CT image.

With CT scanning, several x-ray beams and electronic x-ray detectors rotate around you. These measure the amount of radiation being absorbed throughout your body. Sometimes, the exam table will move during the scan, so that the x-ray beam follows a spiral path. A special computer program processes this large volume of data to create two-dimensional cross- sectional images of your body. These images are then displayed on a monitor. CT imaging is sometimes compared to looking into a loaf of bread by cutting the loaf into thin slices. When the image slices are reassembled by computer software, the result is a very detailed multidimensional view of the body's interior.

Refinements in detector technology allow nearly all CT scanners to obtain multiple slices in a single rotation. These scanners, called multi-slice or multidetector CT, allow thinner slices to be obtained in a shorter amount of time. This results in more detail and additional view capabilities.

Modern CT scanners can scan through large sections of the body in just a few seconds. Such speed is beneficial for all patients who find it difficult to stay still, even for the brief time necessary to obtain images.

For some CT exams, a contrast material is used to enhance visibility in the area of the body being studied.

HOW IS THE PROCEDURE PERFORMED?

The technologist begins by positioning you on the CT exam table, usually lying flat on your back. Straps and pillows may be used to help you maintain the correct position and remain still during the exam.

Motion will cause blurring of the images and degrade the quality of the examination the same way that it affects photographs.

If contrast material is used, depending on the type of exam, it will be swallowed, injected through an intravenous line (IV) or, rarely, administered by enema.

Next, the table will move quickly through the scanner to determine the correct starting position for the scans. Then, the table will move slowly through the machine as the actual CT scanning is performed.

Depending on the type of CT scan, the machine may make several passes.

You may be asked to hold your breath during the scanning. Any motion, including breathing and body movements, can lead to artifacts on the images. This loss of image quality can resemble the blurring seen on a photograph taken of a moving object.

When the exam is complete, you will be asked to wait until the technologist verifies that the images are of high enough quality for accurate interpretation.

The CT examination is usually completed within 30 to 60 minutes. The portion requiring intravenous contrast injection usually lasts only 10 to 30 seconds.

WHAT WILL I EXPERIENCE DURING AND AFTER THE PROCEDURE?

CT exams are generally painless, fast and easy.

Though the scan is painless, you may have some discomfort from remaining still for several minutes or from placement of an IV. If you have a hard time staying still, are very nervous, anxious or in pain, you may find a CT exam stressful.

If an intravenous contrast material is used, you will feel a pin prick when the needle is inserted into your vein. You may feel warm or flushed while the contrast is injected. You also may have a metallic taste in your mouth. This will pass. You may feel a need to urinate. However, this is a contrast effect and subsides quickly.

If the contrast material is swallowed, you may find the taste mildly unpleasant; however, most patients can easily tolerate it. You can expect to experience a sense of abdominal fullness and an increasing need to expel the liquid if your contrast material is given by enema. In this case, be patient, as the mild discomfort will not last long.

When you enter the CT scanner, you may see special light lines projected onto your body. These lines are used to ensure that you are properly positioned. With modern CT scanners, you may hear slight buzzing, clicking and whirring sounds. These occur as the CT scanner's internal parts, not usually visible to you, revolve around you during the imaging process.

You will be alone in the exam room during the CT scan. However, the technologist will always be able to see, hear and speak with you through a built-in intercom system.

After a CT exam, the technologist will remove the intravenous line used to inject the contrast material.

The tiny hole made by the needle will be covered with a small dressing. You can return to your normal activities.

WHO INTERPRETS THE RESULTS AND HOW DO I GET THEM?

A radiologist, a doctor specially trained to supervise and interpret radiology exams, will analyze the images. The radiologist will send an official report to the doctor who ordered the exam.

Follow-up exams may be needed. If so, your doctor will explain why. Sometimes a follow-up exam is done because a potential abnormality needs further evaluation with additional views or a special imaging technique. A follow-up exam may also be done to see if there has been any change in an abnormality over time. Follow-up exams are sometimes the best way to see if treatment is working or if an abnormality is stable or has changed.

WHAT THE BENEFITS VS. RISKS?

BENEFITS

• CT scanning is painless, noninvasive and accurate
• A major advantage of CT is its ability to image bone, soft tissue and blood vessels all at the same time
• Unlike conventional x-rays, CT scanning provides very detailed images of many types of tissue as well as the lungs, bones, and blood vessels
• CT examinations are fast and simple; in emergency cases, they can reveal internal injuries and bleeding quickly enough to help save lives
• CT has been shown to be a cost-effective imaging tool for a wide range of clinical problems
• CT is less sensitive to patient movement than MRI
• CT can be performed if you have an implanted medical device of any kind, unlike MRI
• CT imaging provides real-time imaging, making it a good tool for guiding minimally invasive procedures such as needle biopsies and needle aspirations of many areas of the body, particularly the lungs, abdomen, pelvis and bones
• A diagnosis determined by CT scanning may eliminate the need for exploratory surgery and surgical biopsy
• No radiation remains in a patient's body after a CT examination
• X-rays used in CT scans should have no immediate side effects

RISKS

There is no conclusive evidence that radiation at small amounts delivered by a CT scan causes cancer.

Large population studies have shown a slight increase in cancer from much larger amounts of radiation, such as from radiation therapy. Thus, there is always concern that this risk may also apply to the lower amounts of radiation delivered by a CT exam. When a CT scan is recommended by your doctor, the expected benefit of this test outweighs the potential risk from radiation. You are encouraged to discuss the risks versus the benefits of your CT scan with your doctor or radiologist, and to explore whether alternative imaging tests may be available to diagnose your condition.

• The effective radiation dose for this procedure varies
• Women should always tell their doctor and x-ray or CT technologist if there is any chance they are pregnant
• CT scanning is, in general, not recommended for pregnant women unless medically necessary because of potential risk to the unborn baby
• Mothers should not breastfeed their babies for 24 hours after contrast material is given. Studies show the amount of contrast absorbed by the infant during breastfeeding is extremely low
• The risk of serious allergic reaction to contrast materials that contain iodine is extremely rare, and radiology departments are well-equipped to deal with them

WHAT ARE THE LIMITATIONS OF CT SCANNING?

Soft-tissue details in areas such as the brain, internal pelvic organs, and joints (such as knees and shoulders) can often be better evaluated with magnetic resonance imaging (MRI). In pregnant women, while CT can be performed safely, other imaging exams not involving radiation, such as ultrasound or MRI, are preferred but only if they are likely to be as good as CT in diagnosing your condition.

A person who is very large may not fit into the opening of a conventional CT scanner or may be over the weight limit”usually 200 kg”for the moving table.